1. Field of the Invention
The present invention relates to a filter used for optically reading an image from an original in an image reading device.
2. Description of the Related Art
An image reading device installed in a color image processing apparatus such as a color facsimile machine, a color scanner, a color copier, or a color multifunction peripheral generally includes an optical unit for reading images from originals. The optical unit is typically provided with light emitting elements that emit lights with wavelengths corresponding to RGB colors (red, green, and blue), respectively, to support color originals in predetermined sizes. Such light emitting elements emit lights for RGB at different emission timings, respectively, and the emitted lights are applied as reading lights to an original. The reading lights are then reflected from the original and the reflected lights are input into a photodetecting unit in the optical unit to form an image. The photodetecting unit outputs an electrical signal corresponding to an image read from the original.
Generally, LED (Light Emitting Diode) devices are employed as light emitting elements in optical units of color image reading devices, which is disclosed in, for example, Japanese Patent Application Laid-open No. 2004-144859.
One characteristic of a typical LED device is that an optical wavelength range of spectral characteristics of an LED depends on variation in manufacturing. Therefore, if such an LED is used for reading an image from an original, image characteristics of the read image depends on the LED. To address the above problem, for example, Japanese Patent Application Laid-open No. 2005-33834 discloses a conventional technology for performing various image processing on read image data.
However, the core technique of the conventional technology is to reduce effects due to variation in manufacturing of LEDs, and leaves other factors that may degrade precision of optical systems unresolved. In other words, demands for using reflected lights from originals with good precision, including improvement in image reading precision, are growing.
Specifically, because even the same type of LEDs emit lights with different optical wavelengths, even when originals in a color whose spectral characteristics is uniform are irradiated with lights from the same type of LEDs, electrical signal levels obtained by an optical-to-electrical (OE) conversion using reflected lights or transmitted lights from the originals (i.e., outputs from an image sensor) depend on a range of the variation in the optical wavelengths of the LEDs and effects of the spectral characteristics of the originals. As a result, quality of read images may be degraded.